The present invention relates more particularly, but not exclusively, to a type of pump that is commonly known as a “pusher-pump”. Such a term is explained by the fact that the dispenser member includes a pusher that forms not only a dispenser orifice, but also defines a portion of a liquid chamber in which the liquid is put under pressure in selective manner. In some pumps, an inner surface of the pusher, of generally substantially cylindrical shape, serves as a sealing slide-cylinder for a piston of an outlet valve that is displaced with sealing contact inside the cylinder, thereby uncovering the dispenser orifice in selective manner. In general, the pistons are of the differential type, being displaced in response to a variation in the pressure of the liquid inside the chamber. Thus, in such a pusher-pump, there is a valve piston and a main piston that are displaceable in sealing contact in respective cylinders. The two pistons can be made integrally, and the unit as a whole can be referred to simply by the term “piston” comprising a main-piston lip and an outlet valve lip.
In the prior-art, documents WO 97/23304, U.S. Pat. No. 4,050,613, and WO 2005/063405 are known that all describe pusher-pumps that operate on the above-defined principle. They all describe pumps comprising a pusher, a body that is mounted securely by means of a ring on the opening of a receptacle, and a differential piston that integrates the main-piston and valve-piston functions by forming a main-piston lip and one or two outlet valve lips. The differential piston slides inside the pusher in response to a variation in pressure. The body, the pusher, and the differential piston together form a chamber. When the pressure increases in the chamber, the differential piston is displaced relative to the pusher. In addition, the inlet valve of the chamber is formed by a ball, a deformable-flap valve, or by the differential piston itself.
The problem that is encountered with that type of pump is priming the pump, i.e. filling the chamber for the first time with liquid coming from the reservoir. The above-mentioned prior-art documents do not deal with that problem. Unlike conventional dispensers in which the pump makes it possible to expel the air that is initially contained in the chamber to the inside the reservoir, very often this is not possible with pusher-pumps, since they are mounted on reservoirs of very small capacity. As a result, it is not possible to expel the air that is initially contained in the chamber into the reservoir, since the reservoir is completely full of liquid. Expelling the air into a reservoir of small capacity could result in the pump malfunctioning because of the raised pressure of the liquid stored in the reservoir. Consequently, the prior-art solution cannot be applied to small-capacity reservoirs, such as reservoirs on which pusher-pumps are generally mounted. More generally, an object of the present invention is to prime a liquid dispenser device in simple manner, without an additional step, and at low cost. The solution consisting in expelling the air into the reservoir is not possible.